1. Field of the Invention
The present invention relates to a low thermal expansion cordierite aggregate useful for a filter for a high temperature dust-containing gas and its bonded body.
2. Discussion of Background
A cordierite ceramics has not only a refractoriness of at least 1300.degree. C. but also a small thermal expansion coefficient and an excellent thermal spolling resistance. By virtue of these properties, it is used as a honeycomb catalyst carrier for cleaning an exhaust gas from an automobile. On the other hand, application of a cordierite ceramics to a filter for removing particulates from a diesel exhaust gas and for removing dusts from a high temperature coal combustion flue gas has been studied.
As general methods for producing cordierite ceramics, a firing method or a glass ceramic method is adopted. In the former method, a molded body made of powders such as clay, talc and alumina powders is fired to form cordierite crystals by simultaneous sintering and a solid phase reaction. In the latter method, glass particles obtained by the sol-gel method or by the melt cooling method is crystallized to obtain cordierite aggregates, and the cordierite aggregates obtained are used as a main starting material and, such material is molded and bonded (here the term "bonded" means not only sintered but also solidified with a binder such as a cement) are known.
There are two cordierite crystal phases, one is an .alpha.-type (a hexagonal system) and the other is a .beta.-type (an orthorhombic system). It is known that either cordierite crystal has negative thermal expansion in the direction of the c-axis and about the same positive thermal expansion in the directions of the other axes.
A mean thermal expansion coefficient within a temperature range of from room temperature to 1000.degree. C. (hereinafter referred to as a mean thermal expansion coefficient) of a cordierite ceramics obtained by a conventional method is usually within a range of from 20.times.10.sup.-7 to 25.times.10.sup.-7 /.degree.C., which is almost equal to the arithmetic mean of the above-mentioned thermal expansion coefficients in the directions of respective crystallographic axes.
It is also known that when powder materials consisting of flaky particles of cleavable crystals such as clay minerals are extruded into a honeycomb form and fired, a sintered body will be obtained wherein the c-axes of the cordierite crystals, in which direction the cordierite crystals have a negative thermal expansion coefficient, are oriented in parallel to the wall surface of the extruded honeycomb. And the sintered body shows a small mean thermal expansion coefficient of at most 14.times.10.sup.-7 /.degree.C., occasionally of at most 5.6.times.10.sup.-7 /.degree.C. in the direction parallel to the wall surface ("Ceramics", Vol 14, No. 11, pp. 967-976, 1979).
Japanese Examined Patent Publication No. 20269/1982 proposes a method for producing a low thermal expansion cordierite having a mean thermal expansion coefficient of at most 18.times.10.sup.-7 /.degree.C. wherein glass particles having an approximate cordierite composition and a particle size of at least 1 mm are heated to 1320.degree.-1410.degree. C. to crystallize them into cordierite. In Examples of the Patent Publication, a cordierite aggregate having a mean thermal expansion coefficient as small as 13.times.10.sup.-7 /.degree.C. is obtained.
The cordierite ceramics produced by the method disclosed in the Patent Publication has little orientated cordierite crystal structure and has a mean thermal expansion coefficient remarkably smaller than other cordierite ceramics having no orientation in its crystal structure. However, a cordierite aggregate or a cordierite bonded body having a means thermal expansion coefficient of 13.times.10.sup.-7 /.degree.C. can not be produced reproducibly by the method disclosed in the Patent Publication.
For production of a cordierite ceramics so-called crystallized glass, a method is employed wherein a component which forms crystal nuclei, such as ZrO.sub.2, is incorporated into the starting material, and a glass body made of such material is maintained at a nucleation temperature to form many crystal nuclei and then maintained at such an elevated temperature so that the crystal nuclei grow, and it is converted to a crystallized glass composed of many fine crystals.
However, the crystallized glass obtained by this method has a conventional thermal expansion coefficient and it is composed of fine cordierite crystals, and crystals of mullite, proto-enstatite and cristobalite are liable to be formed during the crystallization.
When a cordierite ceramics filter in a form other than a honeycomb form such as a thick-walled cylinder is used for removing dust from a high temperature coal combustion flue gas, it is likely that flammable materials in the collected dust suddenly burn, and the resulting sudden rise in the temperature of the dust-containing gas gives the filter a serious thermal shock. In such a case, a cordierite ceramic filter having a mean thermal expansion coefficient larger than 13.times.10.sup.-7 /.degree.C. is not necessarily adequate in respect of thermal spolling resistance, and there has been a problem that the filter cracks or breaks whereby the whole system has to be suspended.
Namely, with respect to conventional cordierite ceramics, only when a honeycomb is extruded to orientate the c-axis of the crystals, a bonded body having a small mean thermal expansion coefficient of at most 10.times.10.sup.-7 /.degree.C. can be obtained. With respect to a thick-walled cordierite ceramics, a practical bonded body having a small mean thermal expansion coefficient of about 10.times.10.sup.-7 /.degree.C. has not been known yet.
Japanese Unexamined Patent Publication No. 111659/1990 proposes a method for preparing a low thermal expansion cordierite composed of a single phase of .alpha.-cordierite, wherein a mixture having a cordierite composition is heated to melt and then cooled at a rate of less than 1.5.degree. C./second to a level of from 900.degree. to 1100.degree. C. and maintained at this temperature. However, the method has a tendency to accompany crystallizations of mullite, proto-enstatite and cristobalite which enhance thermal expansion coefficient of cordierite ceramics.
The Unexamined Patent Publication also reports that cordierite having a mean thermal expansion coefficient as small as 8.8.times.10.sup.-7 /.degree.C. is obtained by crystallizing a melt containing 5% by weight of ZrO.sub.2 as a crystal nucleating agent at 1000.degree. C. for 4 days. However, it is reasonable to presume that the cordierite has a crystal structure much smaller than 50 .mu.m due to addition of the nucleating agent, and it is not clear why the cordierite has such a small mean thermal expansion coefficient.
The measured values are likely to include considerable errors, since alumina, which has a large mean thermal expansion coefficient, is used as a reference specimen in the measurement of the mean thermal expansion coefficient. In any case, the method which requires to maintain the melt at 1000.degree. C. for a period as long as 4 days (96 hours) lacks in practicability.